Uhm, if you mean a 100Mb HD and 4Mb RAm, then you're plain outta luck for the "newbie friendly" part if you mean graphical user interface oriented. No way an X server will run in 4Mb, I think bare requirements say 16Mb, tho that will be very slow.
As for size, if you've got a cdrom you could try the Finnix cd (400Mb), Zipslack (tho thats for Iomega Zipdrives but since they are 100Mb...), Trinux' iso (25Mb) or grab the Trinux floppy distro files, else go for a small 1 or 2 Floppy distro like Pocket Linux/Nuclinux/Tomsrtbt/Loaf and build it up from that. Hmm. Maybe a bit daunting task for a newbie, but its all I can come up with at the mo.